1. Field of the Invention
The invention relates to glass break detectors for alarm systems. More particularly, but not by way of limitation, this invention relates to multiple filter glass break detectors for alarm systems.
2. Description of the Related Art
It has become common to use alarm systems for providing security to commercial buildings, homes, automobiles and other areas. Many of these structures have windows which cause more of a problem than do doors and walls for an alarm system which provides security for the structure. In order to overcome the problems caused by the windows, glass break detectors have been developed.
Some of the types of glass break detectors include active detectors, physical vibration detectors and acoustic detectors. Active glass break detectors send a specific frequency energy to or through the glass and monitors the return of that energy for changes therein to determine if the glass has been broken. Physical vibration detectors have a sensing unit mounted directly on the glass and monitor the glass for vibrational energies which indicate that the glass has been broken. Acoustic detectors monitor sound waves for specific frequencies and amplitudes which relate to frequencies and amplitudes of broken glass.
Generally, glass break detectors sense the higher level of energy briefly emitted by the breaking of glass. However, background noise, such as aircraft flying nearby or other loud sources of noise, can produce levels of energy which the glass break detectors can mistakenly sense as breaking glass. To reduce false alarms caused by background noise, glass break detectors have incorporated filters for selectively passing frequencies resulting from breaking glass.
By selecting a single frequency produced by breaking glass, the glass break detector could be mounted in a location, or general ambient conditions could be such that, the glass break detector would have more of a tendency to false alarm because the single frequency or band can occur in sounds other than breaking glass. Glass break detectors have overcome this deficiency by using multiple filters to provide amplitude monitoring for a plurality of frequencies. These plurality of frequencies are selected according to the most critical frequencies produced by breaking glass.
The amplitude of the frequencies selected are compared to threshold values to determine if the noise produced resulted from breaking glass. These threshold values are predetermined and preset values. Because these values are predetermined and preset, they are unable to compensate for varying ambient background noise conditions or the different critical frequencies of various types of breaking glass. Each type of glass has its own unique frequencies and amplitudes that it produces when it breaks. Therefore, the preset threshold values of the prior art glass detectors must be uniquely set for the type of glass that the glass break detector is to sense. This presents a limitation for each unique glass break detector on its application to types of glass.
In order to set threshold values, it is necessary to determine the critical frequencies and amplitudes of the glass that is being monitored. In prior art glass break detectors, the critical frequencies and amplitudes of the glass to be sensed for breakage was monitored by a sensing device which is unrelated to the actual glass break detector. By using an external device to monitor the characteristics of breaking glass, the threshold values are set in the glass break detector without regard to the individual characteristics of how the glass break detectors senses the characteristics of the glass or how the external detection device senses the characteristics of breaking glass. This blind setting of threshold values leaves an uncertainty as to what the glass break detector is actually sensing or where the glass break detector is actually set in regard to the threshold value.
In sensing the select frequencies of breaking glass, the prior art glass break detectors use comparators that continuously monitor each filter. If, for processing purposes, it is desired to use an analog-to-digital converter for changing the frequency data to digital form, this would require multiple analog-to-digital converters. The use of multiple analog-to-digital converters presents a problem of complex and more expensive circuitry.
The passive acoustic glass break detectors depend on sensing acoustic noise transmitted from the breaking glass across the ambient conditions and to an acoustic sensor. Factors in the ambient conditions such as distance from the breaking glass to the glass break sensor can cause the acoustic noise transmitted by a breaking glass to diminish such that the sensor is unable to pick up enough noise signal for the glass break detector circuitry to sense the breaking glass. One means of compensating for diminished acoustic signals is by providing a preamp immediately after the sensor for amplifying the level of sound sensed. However, this requires extra power and circuitry for the glass break detector.
For the foregoing reasons, there is a need for a glass break detector which can reduce the number of false alarms but yet still pick up breaking glass with more reliability, does not need to rely on preset threshold values for determining when glass has been broken and can monitor and display what the glass break detector actually senses and at what threshold that the glass break detector triggers.